fix(merge_utils): normalize string-shorthand entries in merge_module_lists

[bkrabach]

Summary

Fixes AttributeError: 'str' object has no attribute 'get' raised at
merge_utils.py:62 (and symmetrically in the base loop) when a
hooks / tools / providers list contains string-shorthand entries
instead of dict-shape entries.

Root cause

String-shorthand is intentional — agent .md files declare module
dependencies using a bare YAML list of strings, e.g.:

# reality-check/agents/terminal-tester.md, line 38
tools: [terminal_inspector]

rather than the full dict form:

tools:
  - module: terminal_inspector

The YAML parser returns ["terminal_inspector"] (a list[str]). When
this agent config is overlaid onto the parent session config,
merge_agent_dicts routes the tools key to merge_module_lists,
which unconditionally called .get() on each list element — crashing
immediately for strings.

This pattern is used across multiple bundles:

• reality-check/agents/terminal-tester.md → tools: [terminal_inspector]
• terminal-tester/agents/terminal-operator.md → tools: [terminal_inspector]
• terminal-tester/agents/terminal-visual-tester.md → tools: [terminal_inspector]
• terminal-tester/agents/terminal-debugger.md → tools: [terminal_inspector]

The fix belongs in merge_module_lists: normalize at the merge boundary,
not in every caller or every YAML author. No upstream caller change is
needed because all authors are writing intentional shorthand, not bugs.

Fix

Added _normalize_module_entry() helper that:

• Converts str → {key_field: <string>} (e.g. "bash" → {"module": "bash"})
• Passes dict through unchanged
• Returns None for anything else (silently dropped, rather than raising)

merge_module_lists now applies this normalization to both base
and overlay before any .get() calls, via a single comprehension pass.
The type signature is widened from list[dict[str, Any]] to list[Any]
on both parameters (return type remains list[dict[str, Any]]).

Why this is hard to find

The exception is caught at _execute_recipe's broad except Exception as e:
in amplifier-bundle-recipes/__init__.py:464, which converts it to a string
without the traceback. Resolving this required reproducing in a manual
interactive amplifier tool invoke recipes invocation with the outer except
patched to print traceback.format_exc(), in order to find the actual line
in merge_utils.py:62.

PR microsoft/amplifier-bundle-recipes#71 (isinstance(agent_cfg, dict)
guards) addressed a similar class of bug but in executor.py:1785/1803 —
two-and-a-half stack frames upstream of this one. The defense-in-depth
pattern needs to be applied throughout the merge chain.

Test

Adds TestMergeModuleListsStringShorthand with 13 cases covering:

Test	Scenario
test_string_overlay_single_entry	overlay=["bash"] → [{"module": "bash"}]
test_string_overlay_multiple_entries	Multiple strings normalized
test_string_overlay_merges_into_existing_base	String normalised, merges into matching dict base
test_string_overlay_appends_new_entry	String not in base is appended
test_mixed_overlay_strings_and_dicts	Mixed overlay handled correctly
test_string_base_single_entry	base=["bash"] → [{"module": "bash"}]
test_string_base_multiple_entries	Multiple string base entries
test_mixed_base_strings_and_dicts	Mixed base handled correctly
test_string_base_merged_with_dict_overlay	String base + dict overlay deep-merges
test_real_world_terminal_inspector	Exact regression: ["terminal_inspector"]
test_key_field_respected_for_string_normalization	key_field param propagates
test_empty_lists	Empty lists → no crash
test_garbage_entries_silently_dropped	None, int dropped gracefully

Upstream fix

No upstream fix was needed — the strings come from intentional YAML authoring
conventions, not from a serialization bug. The normalization at the merge
boundary is the correct and complete fix.

Discovered while

Running the amplifier-bundle-reality-check recipe inside an Amplifier-Resolve
reality-check runner sub-container. Step 2 (terminal-validation) consistently
failed in ~11ms because spawning the reality-check:terminal-tester agent
(which declares tools: [terminal_inspector]) crashed in merge_module_lists
before the agent session could start.

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